When Shi'ite Muslim fighters defeated presidential guards and tightened their grip on Sana'a this week, it cemented the role of their shrewd, practical young leader Abdel-Malek al-Houthi as Yemen's predominant power broker.

His Houthi movement, founded by his elder brother over a decade ago and named for the family, has controlled the capital since September after winning a series of battles with army units and Sunni tribes in its northern heartland.

Yet despite the Houthis' effective dominance of Yemen in recent months, little is known of their commander, who rarely appears in public or gives press interviews, but whose few speeches reveal a gift for political opportunism.

Zaydi Shi'ites

The group was formed to fight for the interests of the Zaydi Shi'ites, a minority sect that ruled a 1,000-year kingdom in Yemen until 1962 but considered themselves threatened under long-serving ruler Ali Abdullah Saleh, himself a Zaydi.

The fighters took on the family's name after Abdel-Malek's brother Hossein al-Houthi was killed in battle against Saleh's forces in 2004.

Their senior leadership remains shrouded in obscurity, while their ultimate goals, willingness to compromise on ideology and links to Shi'ite power Iran remain a source of nervous speculation in the West and neighboring Gulf states.

Abdel-Malek al-Houthi, in his early 30s, originally won his reputation as a tough, efficient battlefield commander in the group's series of six wars as a rebel movement fighting Saleh's forces.

But since mass protests in 2011, he has positioned himself as a revolutionary national leader, claiming the mantle of the demonstrators who flocked Sana'a's streets four years ago demanding an end to corruption and dictatorial government.

After Houthi forces and allied militias calling themselves the "popular committees" took control of Sana'a four months ago, he claimed no formal position, but forced President Abd-Rabbu Mansour Hadi to install a new government.

Displayed across Sana'a, the group's slogan "Death to America, Death to Israel" was modeled on Iran's revolutionary motto, and many Yemenis drew parallels between the Houthis and another of Iran's Shi'ite proteges -- Lebanon's Hezbollah.

Leaders share similarities

That comparison extends to their leaders, with analysts pointing to similarities between Abdel-Malek al-Houthi and Hezbollah's Hassan Nasrallah in terms of how they use media and even how they structure their speeches.

While Hezbollah's close relationship with Iran is firmly established, less is known about Houthi reliance on Tehran, although there is a widespread impression that it is growing.

Yemeni and Iranian officials said Iran supplied military and financial support to Houthi forces both before and after their takeover of Sana'a. A senior Houthi official denied this.

A senior Iranian official told Reuters last year the pace of money and arms getting to the Houthis had increased since their seizure of Sana'a.

Nevertheless, Zaydi Shi'ism is very different to the version practiced in either Lebanon or Iran and the Houthi leader has shown himself highly pragmatic, rather than ideologically driven, in making political alliances.

The September capture of Sana'a is widely thought to have involved a measure of cooperation with former president Saleh, a once intractable enemy of the Houthis, and a sign of the group's ideological fluidity.

'Ability as a commander'

That practical approach to politics may have been learnt by Abdel-Malek during his long years as a guerrilla commander, when he won the support of his troops by promoting commanders based on results instead of their status or ideology.

"His ability as a commander is what has made him popular," said Mohammed al-Bukhaiti, a longtime supporter of the movement.

His military experience may also have been responsible for his ultra-cautious approach to security. He is known for rarely staying long in one place, for never meeting press and for an extreme reluctance to make scheduled public appearances.

Even his inner circle -- bar one or two names -- is kept a secret. That is because he "does not want them to get hurt," said Mohammed Azzem, a noted Yemeni intellectual who has known Abdel-Malek al-Houthi since the leader was a child.

Only 11 years ago there was nothing to suggest the then-23-year-old would so quickly become the most powerful man in a country where experience and seniority were traditionally the main qualifications for leadership.

His father, Badr al-Din al-Houthi, a Zaydi scholar, had founded a religious revivalist group to stop conservative Sunni ideas, funded from Saudi Arabia, spreading in countryside around the family's poor northern city of Saadeh.

Abdel-Malek, the youngest of Badr al-Din's eight sons, was then a quiet, hardworking religious student, a "simple guy," who ran errands for his elderly father, Azzem said.

Instead, it was an older brother Hossein, a member of parliament, who built the movement into a political and military force, turning to arms after Saleh's government put a price on his head and sent the army to Saadeh.

Saleh accused the Houthis of trying to overturn Yemen's 1962 revolution and rebuild the Zaydi imamate that had ruled the highlands for a thousand years -- something many of the country's Sunnis still fear.

When Hossein was killed in battle in 2004 the baton was passed to his father Badr al-Din and then, when he died in 2005, to Abdel-Malek.